Microwave radiation is used for drying honeycomb structures that are formed by extrusion and used for a variety of applications such as engine filters, catalytic converters, and the like. In comparison with conventional heat-based oven drying, microwave drying provides a higher drying rate and is generally faster because the honeycomb structure or “log” is heated directly through the interaction of the microwave energy with the water in the log.
Microwave drying is carried out in a microwave dryer that includes at least one applicator, and that often has a series of applicators, e.g., two or three. A portion of the microwave radiation introduced into a given applicator is absorbed (dissipated) in the log during the drying process. The amount of microwave power dissipation is generally proportional to the water (moisture) content in the log. For example, a wet log (e.g., a newly extruded log) will generally absorb more power than a dry log. During the drying process, the microwave radiation that is not absorbed by the honeycomb structure is either absorbed by other materials in the applicator or reflected back to the generator and therefore does not contribute to the drying process. A large amount of reflected microwave radiation can cause throughput reduction, inefficiency in the manufacturing process, and damage to the microwave radiation source (e.g., magnetron).
To have an efficient microwave process, it is desirable to keep the amount of reflected microwave power within a given applicator to within an acceptable limit or threshold, e.g., less than about 20% of the output power. Toward the end of the drying process when the logs are nearly dry and nearly ready to exit the applicator, the applicator system can reflect a large amount of microwave power. Consequently, to maintain the amount of reflected microwave power within an acceptable limit, the amount of microwave radiation (power) needs to be reduced. While effective, this approach leads to the underutilization of the applicator.